A solstice and an equinox are astronomical events that mark the beginning of the four seasons. Defined by Earth’s position relative to the Sun, these markers occur twice each year, signaling a shift in the amount of solar energy different regions receive. Understanding the difference requires recognizing the specific orientation of the Earth’s axis at that moment.
The Role of Axial Tilt
The constant tilt of Earth’s rotational axis is the underlying cause for both solstices and equinoxes, and ultimately for the planet’s seasons. Earth’s axis is perpetually tilted by approximately 23.5 degrees relative to the plane of its orbit around the Sun (the ecliptic plane). This tilt is fixed, always pointing toward the same area of space, near the North Star, Polaris.
As the planet travels along its annual path, the orientation of this fixed tilt changes relative to the Sun. This causes the hemispheres to alternately lean toward or away from the solar rays. This variation dictates how directly sunlight hits a particular latitude, controlling the intensity of solar radiation and the length of daylight hours.
Solstices Maximum Tilt and Daylight Extremes
Solstices are the moments when the Earth’s axial tilt is at its maximum angle toward or away from the Sun, resulting in the most extreme exposure for one hemisphere. This extreme tilt causes the Sun to appear at its highest or lowest elevation in the sky.
The June solstice (around the 21st) is the Summer Solstice in the Northern Hemisphere, marking the longest day and shortest night because that hemisphere is maximally tilted toward the Sun. Conversely, the December solstice (near the 21st) is the Winter Solstice, marking the shortest day and longest night because the hemisphere is tilted maximally away.
On the summer solstice, the Sun’s most direct rays hit the Tropic of Cancer (23.5 degrees North latitude). On the winter solstice, the Sun is directly overhead at the Tropic of Capricorn (23.5 degrees South latitude). The word “solstice” comes from the Latin solstitium, meaning “sun standing still,” as the sun’s apparent movement pauses before reversing direction.
Equinoxes Zero Tilt and Equal Day and Night
Equinoxes occur when the Earth’s axis is positioned sideways, meaning it is neither tilted toward nor away from the Sun relative to the incoming solar rays. At this moment, the axis is perpendicular to the line connecting the Earth and the Sun, resulting in the Sun being positioned directly over the Equator. This alignment ensures that both the Northern and Southern Hemispheres receive nearly equal amounts of sunlight.
This zero tilt results in approximately equal hours of daylight and darkness across nearly all latitudes worldwide. The Vernal or Spring Equinox occurs around March 20th, and the Autumnal Equinox occurs around September 22nd, marking the transition into spring and autumn. On the day of an equinox, the Sun rises due east and sets due west everywhere on Earth.